Gas exchange is the process by which oxygen and carbon dioxide move between the bloodstream and the lungs. This is the primary function of the respiratory system. It is essential to ensure a constant supply of oxygen to tissues, as well as removing carbon dioxide to prevent its accumulation.
This article will discuss the principles of gas exchange, factors affecting the rate of exchange, and relevant clinical conditions.
Physics of Gas Diffusion
The movement of gases in a contained space (in this case, the lungs) is random. However, overall diffusion results in movement from areas of high concentration to those of low concentration. The rate of diffusion of a gas is primarily affected by
- Concentration gradient: The greater the gradient, the faster the rate
- Surface area for diffusion: The greater the surface area, the faster the rate
- Length of the diffusion pathway: The greater the length of the pathway, the slower the rate
Collision of the molecules of gas with the sides of the container results in pressure. This is defined by the ideal gas law, given in the following equation:
(n represents the number of moles, R the gas constant (8.314), T the absolute temperature and V the volume of the container)
Diffusion of Gases Through Gases
When gases are diffusing through other gases (such as in the alveoli), their rate of diffusion can be defined by Graham’s Law:
“The rate of diffusion is inversely proportional to the square root of its molar mass at identical pressure and temperature”
In other words, the smaller the mass of a gas, the more rapidly it will diffuse.
Diffusion of Gases Through Liquids
When gases are diffusing through liquids, for example across the alveolar membrane and into capillary blood, the solubility of the gases is important. The more soluble a gas is, the faster it will diffuse.
The solubility of a gas is defined by Henry’s law, which states that:
“The amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid”.
If we assume that the conditions of temperature and pressure for all gases remain fixed (as they roughly do in the alveoli) then it is the inherent differences between different gases that determine their solubility.
Carbon dioxide is inherently more soluble than oxygen, and thus diffuses much faster than oxygen into liquid.
Fick’s law gives us a number of factors that affect the rate of diffusion of a gas through fluid:
- The partial pressure difference across the diffusion barrier
- The solubility of the gas
- The cross-sectional area of the fluid
- The distance molecules need to diffuse
- The molecular weight of the gas
- The temperature of the fluid – not important within the lungs and can be assumed to be 37oC
In the lungs, whilst oxygen is smaller than carbon dioxide, the difference in solubility means that carbon dioxide diffuses roughly 20 times faster than oxygen.
This difference between the rate of diffusion of the individual molecules is compensated for by the large difference in partial pressures of oxygen, creating a larger diffusion gradient than that of carbon dioxide.
However, this means that in disease states which impair the ability of the lungs to adequately ventilate with oxygen, oxygen exchange is often compromised before that of carbon dioxide.
Diffusion of Oxygen
The partial pressure of oxygen is low in the alveoli compared to the external environment. This is due to continuous diffusion of oxygen across the alveolar membrane and the diluting effect of carbon dioxide entering the alveoli to leave the body.
Despite this, the partial pressure is still higher in the alveoli than the capillaries, resulting in a net diffusion into the blood. Once it has diffused across the alveolar and capillary membranes, it combines with haemoglobin. This forms oxyhaemoglobin which transports the oxygen to respiring tissues via the bloodstream.
Further information on the transport of oxygen within the blood can be found here.
During exercise, blood spends up to half the normal time (one second at rest) in the pulmonary capillaries due to the increase in cardiac output moving blood around the body more quickly. However, diffusion of oxygen is complete within half a second of the blood cell arriving in the capillary. This means that exercise is not limited by gas exchange.
Diffusion of Carbon Dioxide
The partial pressure of carbon dioxide in the capillaries is much higher than that in the alveoli. This means that net diffusion occurs into the alveoli from capillaries. The carbon dioxide is then exhaled as the partial pressure in the alveoli is higher than the partial pressure in the external environment.
Carbon dioxide is transported in the blood in multiple ways; including dissolved, associated with proteins and as bicarbonate ions. Further information on transport of carbon dioxide in the blood can be found here.
The diffusion barrier in the lungs consists of the following layers:
- Alveolar epithelium
- Tissue fluid
- Capillary endothelium
- Red cell membrane
Factors That Affect The Rate of Diffusion
There are many properties which can affect the rate of diffusion in the lungs. The main factors include:
- Membrane thickness – the thinner the membrane, the faster the rate of diffusion. The diffusion barrier in the lungs is extremely thin, however some conditions cause thickening of the barrier, thereby impairing diffusion. Examples include:
- Fluid in the interstitial space (pulmonary oedema)
- Thickening of the alveolar membrane (pulmonary fibrosis)
- Membrane surface area – the larger the surface area, the faster the rate of diffusion. The lungs normally have a very large surface area for gas exchange due to the alveoli.
- Diseases such as emphysema lead to the destruction of the alveolar architecture. This causes the formation of large air-filled spaces known as bullae. This reduces the surface area available and slows the rate of gas exchange
- Pressure difference across the membrane
- Diffusion coefficient of the gas
Clinical Relevance – Emphysema
Emphysema is a chronic, progressive disease that results in destruction of the alveoli in the lungs. This results in a greatly reduced surface area for gas exchange in the lungs, which typically leads to hypoxia (Type 1 respiratory failure).
The main symptom is of emphysema is shortness of breath, however patients may also experience wheezing, a persistent cough or chest tightness. Emphysema, alongside chronic bronchitis are the conditions that make up Chronic Obstructive Pulmonary Disease (COPD). Whilst smoking is the most common cause, other risk factors include exposure to second-hand smoke, exposure to occupational fumes or dust and living in areas with high levels of pollution.
Treatment depends on the stage of the condition (i.e. the degree of symptoms and airway obstruction) but typically includes:
- Smoking cessation
- Bronchodilators to reduce bronchial constriction
- Inhaled corticosteroids to reduce airway inflammation
- Antibiotics and oral steroids to treat infectious exacerbations of the disease
- Long-term Oxygen Therapy (LTOT) in severe progressive disease